Magnetic Field Sensed by Gene, Study Shows A researcher studying how monarch butterflies navigate has picked up a strong hint that people may be able to sense the earth's Magnetic Field and use it for orienting themselves. Many animals rely on the Magnetic Field for navigation, and researchers have often wondered if people, too, might be able to detect the field; that might explain how Polynesian navigators can make 3,000-mile journeys under starless skies. Since physical chemists had speculated the cryptochromes might be sensitive to magnetism, Dr. Reppert wondered if the monarch butterfly was using its cryptochromes to sense the earth's Magnetic Field. He first studied the laboratory fruit fly, whose genes are much easier to manipulate and showed three years ago that the fly could detect magnetic fields but only when its cryptochrome gene was in good working order. He then showed that the monarch butterfly's two cryptochrome genes could each substitute for the fly's Gene in letting it sense magnetic fields, indicating that the butterfly uses the proteins for the same purpose.



One of the monarch's two cryptochrome genes is similar in its DNA sequence to the human cryptochrome gene. That prompted the idea of seeing whether the human Gene, too, could restore magnetic sensing to fruit flies whose own Gene had been knocked out. The human cryptochrome gene is highly active in the eye, raising the possibility that the Magnetic Field might in some sense be seen, if the cryptochromes interact with the retina. Dr. Reppert said the focus on human use of the Magnetic Field for navigation might be misplaced. Dr. Phillips said that Dr. Reppert's work was of interest but that he had been surprised by an experiment in which Dr. Reppert disrupted the part of the cryptochrome thought to interact with the Magnetic Field, yet the flies had still detected the magnetism.

It's 50-50 whether he's really studying what he thinks he is, Dr. Phillips said. Dr. Reppert replied that he had already ruled out the alternative explanation suggested by Dr. Phillips. Depending on how the proteins are aligned in the eye, insects may perceive objects as being lighter or darker as they orient themselves in relation to the Magnetic Field, Dr. Phillips said. This is the fun stage where we are not constrained by many facts, Dr. Phillips said. As for Dr. Reppert, he is now planning his next step, that of understanding how the cryptochrome proteins sense the Magnetic Field and how they convey that information to the fruit fly's and monarch's brain.

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